Recombinant Borrelia burgdorferi Flagellar biosynthetic protein fliR (fliR)

What is the role of FliR in the flagellar biosynthesis of Borrelia burgdorferi?

FliR is a critical component of the flagellar type III secretion system (T3SS) in Borrelia burgdorferi, functioning as an integral membrane protein in the export apparatus. Similar to other flagellar proteins like FliH and FliI, FliR contributes to the assembly and function of the flagellar structure. The T3SS is responsible for the export and assembly of flagellar structural proteins, which are essential for B. burgdorferi motility and pathogenesis . FliR specifically helps form the membrane-embedded portion of the export apparatus and facilitates the passage of flagellar proteins through the cytoplasmic membrane.

How does FliR interact with other flagellar proteins in the type III secretion system?

FliR works in concert with other flagellar proteins to form a functional export apparatus. While specific interactions of FliR in B. burgdorferi have not been fully characterized, studies of FliH and FliI demonstrate that flagellar proteins form complexes critical for flagellar assembly. The FliH-FliI complex, for instance, is essential for the formation of full-length flagella . FliR likely interacts with other membrane components of the export apparatus (such as FlhA, FlhB, FliO, FliP, and FliQ) to create a channel through which flagellar components are secreted. Disruption of these interactions, as seen with other flagellar proteins, would be expected to compromise flagellar assembly and bacterial motility.

What is the genetic organization of the fliR gene in B. burgdorferi genome?

The fliR gene in B. burgdorferi is part of a flagellar gene cluster on the chromosome. While the search results don't specify the exact location of fliR, studies of flagellar gene organization in B. burgdorferi have shown that flagellar genes are often arranged in operons. For instance, fliH, fliI, and fliJ were shown to be arranged in an operon structure, with mutations in one gene potentially affecting the expression of downstream genes . Researchers investigating fliR should therefore consider potential polar effects when designing gene knockout experiments, and use methods such as RT-PCR to verify the expression of neighboring genes, as was done with the fliH mutant .

What expression systems are most effective for producing recombinant B. burgdorferi FliR?

For effective production of recombinant B. burgdorferi FliR, several expression systems can be considered:

• E. coli-based systems: Similar to the approach used for expressing other B. burgdorferi proteins, such as BbCRASP-2, E. coli offers a convenient platform for heterologous expression . For membrane proteins like FliR, special E. coli strains designed for membrane protein expression (such as C41(DE3) or C43(DE3)) may be preferable.

• B. burgdorferi-based systems: For studying functional aspects, expressing FliR in B. burgdorferi itself or in related species like B. garinii (as was done with BbCRASP-2 ) might provide more native-like protein conformation.

• Complementation approaches: As demonstrated with fliH and fliI mutants, genetic complementation in trans can be used to express recombinant flagellar proteins in B. burgdorferi . This approach allows for verification of protein function in its native context.

Selection of the appropriate expression system should be guided by the specific research questions being addressed, with consideration for protein solubility, activity, and post-translational modifications.

What are the optimal conditions for soluble expression of recombinant FliR?

Obtaining soluble expression of membrane proteins like FliR presents significant challenges. Based on approaches used for other B. burgdorferi proteins:

• Temperature optimization: Lower expression temperatures (16-20°C) often improve solubility of recombinant proteins by slowing the rate of protein synthesis and folding.

• Fusion tags: Addition of solubility-enhancing fusion partners such as MBP (maltose-binding protein), SUMO, or Thioredoxin can improve solubility.

• Detergent screening: For membrane proteins like FliR, incorporation of mild detergents (DDM, LDAO, etc.) during extraction and purification is crucial. A systematic detergent screen should be performed to identify optimal conditions.

• Truncation constructs: Designing constructs that remove predicted transmembrane domains while retaining functional domains may improve solubility.

• Co-expression with chaperones: Co-expressing with molecular chaperones can improve folding and solubility of challenging proteins.

Researchers should validate the functionality of solubly expressed FliR through biochemical or structural assays to ensure that the protein retains its native characteristics.

What purification strategies yield the highest purity and activity of recombinant FliR?

A multi-step purification approach is recommended for obtaining high-purity, active recombinant FliR:

• Affinity chromatography: Utilizing fusion tags (His, GST, MBP) for initial capture. For membrane proteins like FliR, all buffers should contain appropriate detergents to maintain solubility.

• Ion exchange chromatography: To separate FliR from proteins with similar affinity tag binding properties but different charge characteristics.

• Size exclusion chromatography: As a final polishing step to remove aggregates and ensure homogeneity.

• Detergent exchange: If necessary for downstream applications, protein can be transferred to different detergents during purification.

• Quality control: Assess purity by SDS-PAGE and Western blotting, and confirm proper folding using circular dichroism spectroscopy.

For activity assessment, functional assays should be developed based on known or predicted activities of FliR, such as protein-protein interaction studies with other flagellar components.

How can I assess the functionality of recombinant FliR in vitro?

Several approaches can be employed to assess the functionality of recombinant FliR in vitro:

• Protein-protein interaction assays:

  • Pull-down assays using other recombinant flagellar proteins

  • Surface plasmon resonance to measure binding kinetics

  • ELISA-based interaction assays

  • Yeast two-hybrid or bacterial two-hybrid screening

• Structural integrity assessment:

  • Circular dichroism spectroscopy to confirm secondary structure

  • Limited proteolysis to verify proper folding

  • Thermal shift assays to measure protein stability

• Reconstitution experiments:

  • In vitro reconstitution of partial flagellar export apparatus

  • Liposome incorporation assays for membrane proteins

• Complementation studies:

  • Testing whether recombinant FliR can restore function in fliR-deficient bacteria

When validating these assays, it's important to include appropriate controls, such as known functional partners and inactive mutants, similar to the approaches used with other flagellar proteins in B. burgdorferi .

What phenotypes result from fliR mutations in B. burgdorferi?

While the search results don't specifically address fliR mutations, the phenotypes observed in other flagellar protein mutants provide insights into potential fliR mutation effects:

• Morphological changes: Similar to fliH and fliI mutants, fliR mutants would likely display rod-shaped or string-like morphology rather than the typical spiral shape of wild-type B. burgdorferi .

• Motility defects: Severe reduction or complete loss of motility would be expected, comparable to the phenotype of fliH and fliI mutants that showed greatly reduced motility and inability for translational motion in methylcellulose or soft agar .

• Flagellar structure abnormalities: Cryo-electron tomography would likely reveal altered flagellar structures, possibly with truncated flagella or reduced numbers of flagellar assemblies, as observed in fliH and fliI mutants .

• Cell division defects: Elongated cells with incomplete division points might occur, similar to the division defects observed in fliH and fliI mutants .

• Reduced virulence: Impaired infectivity in mouse models would be expected, based on the noninfectivity of fliH and fliI mutants following needle inoculation .

To conclusively determine the phenotypes of fliR mutations, similar experimental approaches to those used for characterizing fliH and fliI mutants would be necessary.

How does temperature affect FliR expression and function in B. burgdorferi?

Temperature is a critical factor affecting gene expression in B. burgdorferi during its transition between tick vector and mammalian host environments:

• Temperature-dependent expression: B. burgdorferi undergoes differential gene expression when transitioning from tick (approximately 23°C) to mammalian host temperature (35-37°C) . While specific data on fliR is not provided in the search results, many flagellar genes show temperature-dependent regulation.

• Combined effects of temperature and blood: Studies have shown that temperature shift combined with blood exposure leads to significant transcriptional changes in B. burgdorferi, affecting numerous genes including those involved in motility and structure . The expression of FliR may similarly be regulated by these environmental cues.

• σ54-dependent regulation: The flagellar gene expression in B. burgdorferi is regulated by the alternative sigma factor σ54, which is required for mammalian infection and vector colonization . FliR expression might be under similar regulatory control, responding to temperature shifts during host transition.

• Experimental approaches: To study temperature effects on FliR expression, researchers can use:

  • Quantitative RT-PCR to measure fliR transcript levels at different temperatures

  • Western blotting to assess protein levels

  • Reporter gene fusions to monitor expression patterns in real-time

  • RNA-seq or microarray analysis to understand fliR regulation in the context of global gene expression changes

Is FliR essential for B. burgdorferi virulence in animal models?

Based on studies of other flagellar proteins, FliR likely plays a crucial role in B. burgdorferi virulence:

• Motility-virulence connection: Flagellar motility is essential for B. burgdorferi to disseminate within hosts and establish infection. Mutations in flagellar genes fliH and fliI resulted in noninfectivity in mice following needle inoculation .

• Host colonization: The ability of B. burgdorferi to colonize and migrate within the tick vector and subsequently infect mammals depends on functional flagella. Defects in the flagellar apparatus would likely compromise these processes.

• Experimental assessment approaches:

  • Needle inoculation of mice with fliR mutants (similar to studies with fliH and fliI mutants)

  • Tick-mouse infection cycle experiments to assess transmission efficiency

  • Tissue culture invasion and persistence assays

  • Monitoring tissue dissemination using bioluminescent B. burgdorferi strains

• Complementation verification: To confirm that any observed virulence defects are specifically due to fliR mutation, genetic complementation should be performed, as was done with fliH and fliI mutants .

It's worth noting that while genetic complementation of fliH and fliI mutants restored morphology and motility, it did not fully restore infectivity , suggesting complex relationships between flagellar gene expression and virulence that would need to be carefully evaluated for fliR as well.

How does FliR contribute to B. burgdorferi motility during infection?

FliR's contribution to B. burgdorferi motility during infection likely includes:

• Export apparatus function: As a component of the flagellar type III secretion system, FliR facilitates the export and assembly of flagellar proteins necessary for motility. Disruption of this process, as seen in fliH and fliI mutants, results in truncated flagella and severely impaired motility .

• Tissue migration capability: FliR function would be crucial for the spirochete's ability to migrate through host tissues, particularly during early dissemination from the tick bite site. Studies using immunofluorescence assays of infected tick tissues, similar to those performed with σ54 mutants , could reveal the importance of FliR for movement within the vector and host.

• Adaptation to different environments: The flagellar apparatus must function in diverse environments during the infectious cycle. FliR may be involved in adapting flagellar function to these changing conditions, similar to how B. burgdorferi alters gene expression in response to blood influx and temperature shifts .

• Quantitative assessment methods:

  • In vitro motility tracking using dark-field microscopy

  • Soft agar penetration assays

  • Microfluidic devices to mimic tissue environments

  • Intravital imaging of fluorescently labeled bacteria in animal models

What interactions occur between FliR and host immune system components?

While direct information about FliR-host immune interactions is not provided in the search results, the following considerations are relevant:

• Flagellin recognition: Bacterial flagellar proteins are recognized by pattern recognition receptors like TLR5. While FliR itself is not an external flagellar component, its role in flagellar assembly indirectly affects the presentation of immunogenic flagellar proteins to the host immune system.

• Immune evasion strategies: B. burgdorferi employs various mechanisms to evade host immunity, including the complement regulator-acquiring surface proteins (CRASPs) . While FliR is not known to directly participate in immune evasion, proper motility facilitated by FliR is crucial for immune avoidance.

• Research approaches to investigate potential interactions:

  • Immunoprecipitation of FliR from infected tissue samples

  • Mass spectrometry to identify binding partners

  • In vitro binding assays with purified host immune components

  • Transcriptomic analysis of immune cells exposed to recombinant FliR

• Vaccination potential: Determining whether recombinant FliR could serve as a potential vaccine target would require assessment of its immunogenicity, conservation across B. burgdorferi strains, and accessibility to antibodies during infection.

What structural studies have been conducted on B. burgdorferi FliR?

Structural studies specific to B. burgdorferi FliR are not detailed in the search results, but several approaches could be applied:

• X-ray crystallography: Challenging for membrane proteins like FliR, but potentially feasible with:

  • Detergent screening to identify stabilizing conditions

  • Lipidic cubic phase crystallization

  • Co-crystallization with antibody fragments

  • Crystallization of soluble domains

• Cryo-electron microscopy (cryo-EM): Particularly valuable for membrane proteins and large complexes:

  • Single-particle analysis of purified FliR or FliR-containing complexes

  • Cryo-electron tomography of flagellar motors, as was used to analyze the effects of fliH and fliI mutations

  • Sub-tomogram averaging to enhance resolution

• NMR spectroscopy: Suitable for structural analysis of smaller domains:

  • Solution NMR of soluble domains

  • Solid-state NMR for membrane-embedded regions

• Computational approaches:

  • Homology modeling based on FliR structures from other organisms

  • Molecular dynamics simulations to study conformational dynamics

  • Protein-protein docking to predict interactions with other flagellar components

These structural studies would provide valuable insights into FliR function and could guide the design of specific inhibitors for potential therapeutic applications.

How can site-directed mutagenesis be used to identify functional residues in FliR?

Site-directed mutagenesis represents a powerful approach to identify functional residues in FliR, similar to the strategy employed for BbCRASP-2 :

• Selection of target residues:

  • Conserved residues identified through sequence alignment across different Borrelia species

  • Residues predicted to be involved in protein-protein interactions

  • Residues in putative functional domains

  • Charged or hydrophobic residues in predicted transmembrane regions

• Mutagenesis strategy:

  • Alanine scanning mutagenesis to neutralize side chain contributions

  • Conservative substitutions to maintain charge or hydrophobicity

  • Creation of double or triple mutants to test additive effects, as done with BbCRASP-2

• Functional assessment:

  • Complementation of fliR mutants with mutated constructs

  • In vitro binding assays with interaction partners

  • Effects on protein stability and folding

  • Impact on flagellar assembly and bacterial motility

• Experimental design considerations:

  • Include controls (wild-type, known inactive mutants)

  • Verify expression levels of mutant proteins

  • Test mutants under various environmental conditions relevant to the Borrelia life cycle

What are the latest techniques for studying protein-protein interactions involving FliR?

Cutting-edge techniques for studying FliR interactions include:

• Proximity labeling approaches:

  • BioID or TurboID fusion to FliR to identify neighboring proteins in vivo

  • APEX2-based proximity labeling with electron microscopy visualization

  • Split-BioID for detecting specific interaction partners

• Advanced microscopy techniques:

  • Super-resolution microscopy (PALM, STORM, STED) to visualize FliR localization

  • Single-molecule tracking to monitor dynamics

  • Förster resonance energy transfer (FRET) to detect interactions

  • Fluorescence recovery after photobleaching (FRAP) to measure mobility

• Mass spectrometry-based techniques:

  • Crosslinking mass spectrometry (XL-MS) to map interaction interfaces

  • Hydrogen-deuterium exchange mass spectrometry (HDX-MS) to identify binding regions

  • Native mass spectrometry of intact complexes

• Microfluidic approaches:

  • Surface plasmon resonance (SPR) for real-time interaction kinetics

  • Microscale thermophoresis (MST) for affinity measurements

  • Single-molecule force spectroscopy to measure binding strengths

• Genetic screening methods:

  • Bacterial two-hybrid screening with B. burgdorferi genomic libraries

  • Suppressor mutation screens to identify functional relationships

  • CRISPR interference screens to identify genetic interactions

These techniques can be applied to understand FliR's role within the flagellar assembly complex and potentially identify novel interaction partners relevant to B. burgdorferi pathogenesis.

How conserved is FliR across different Borrelia species and other spirochetes?

Understanding the conservation of FliR provides insights into its evolutionary importance:

• Sequence conservation analysis:

  • Multiple sequence alignment of FliR from different Borrelia species

  • Identification of conserved domains and residues

  • Calculation of sequence identity and similarity percentages

• Comparative genomics approaches:

  • Analysis of genomic context and organization of the fliR gene

  • Assessment of selection pressure using dN/dS ratios

  • Identification of co-evolving residues that might indicate functional interactions

• Functional conservation:

  • Complementation experiments across species

  • Comparative analysis of flagellar structure and function

  • Assessment of species-specific adaptations

Similar analyses for other B. burgdorferi proteins have revealed important insights. For example, BbCRASP-2 shows high degrees of sequence conservation within B. burgdorferi sensu stricto strains, particularly in regions involved in binding complement factors . Such conservation patterns can indicate functionally important regions of FliR.

What role does FliR play in the adaptation of B. burgdorferi to different hosts?

FliR's potential role in host adaptation can be investigated through several approaches:

• Expression pattern analysis:

  • Quantitative assessment of fliR expression in tick vectors versus mammalian hosts

  • Comparison with known differentially expressed genes during host transition

  • Temporal expression studies during the infection cycle

• Host-specific requirements:

  • Investigation of whether fliR mutations affect colonization of different host species

  • Assessment of the impact on transmission efficiency between vector and host

  • Evaluation of tissue tropism changes in fliR mutants

• Regulatory influences:

  • Analysis of whether fliR expression is regulated by σ54 and σS, key regulators of B. burgdorferi host adaptation

  • Investigation of temperature and blood-meal effects on fliR expression, similar to studies on other B. burgdorferi genes

• Experimental approaches:

  • Development of reporter systems to monitor fliR expression in different host environments

  • Comparative proteomics of wild-type and fliR mutants under host-mimicking conditions

  • In vivo expression technology (IVET) to study fliR expression during infection

Can recombinant FliR be used as a target for new antimicrobial strategies?

Targeting FliR for antimicrobial development presents several possibilities:

• Inhibitor development strategies:

  • High-throughput screening of small molecule libraries against recombinant FliR

  • Structure-based drug design, if structural information becomes available

  • Peptide inhibitors designed to disrupt specific protein-protein interactions

  • Monoclonal antibodies targeting accessible epitopes

• Therapeutic potential assessment:

  • Evaluation of whether FliR inhibition leads to bacterial growth arrest or killing

  • Testing of target specificity across different bacterial species

  • Assessment of resistance development frequency

  • In vivo efficacy studies in animal models of Lyme disease

• Delivery approaches:

  • Conjugation of inhibitors to B. burgdorferi-targeting molecules

  • Nanoparticle encapsulation for improved delivery

  • Cell-penetrating peptides for intracellular delivery

• Combination therapy potential:

  • Synergy testing with existing antibiotics

  • Multi-target approaches addressing different aspects of flagellar function

What methodological challenges exist in studying FliR and how can they be overcome?

Researchers face several challenges when studying FliR:

• Membrane protein expression and purification:

  • Challenge: Obtaining sufficient quantities of properly folded protein

  • Solutions: Optimization of expression systems, fusion partners, and detergent conditions; membrane mimetics like nanodiscs or amphipols

• Genetic manipulation in B. burgdorferi:

  • Challenge: Low transformation efficiency and limited genetic tools

  • Solutions: Optimized electroporation protocols; counterselectable markers; CRISPR-Cas9 adaptation for Borrelia

• Structural analysis:

  • Challenge: Obtaining high-resolution structures of membrane proteins

  • Solutions: Cryo-EM approaches; crystallization of soluble domains; computational modeling

• Functional assays:

  • Challenge: Developing quantitative assays for FliR function

  • Solutions: Reconstitution systems; fluorescence-based interaction assays; in vivo imaging techniques

• Physiological relevance:

  • Challenge: Translating in vitro findings to in vivo significance

  • Solutions: Development of relevant animal models; ex vivo tissue systems; microfluidic devices mimicking host environments

Addressing these challenges requires interdisciplinary approaches and adaptation of techniques from other bacterial systems to the unique biology of B. burgdorferi.

How can recombinant FliR contribute to understanding the mechanism of flagellar assembly in spirochetes?

Recombinant FliR provides a valuable tool for investigating flagellar assembly mechanisms:

• In vitro reconstitution studies:

  • Stepwise assembly of flagellar export apparatus components

  • Assessment of protein-protein interactions and complex formation

  • Investigation of conformational changes during assembly

• Structure-function correlations:

  • Mapping of interaction domains through truncation and mutation analysis

  • Determination of stoichiometry in complexes

  • Visualization of assembly intermediates by electron microscopy

• Comparative analysis with model systems:

  • Assessment of functional conservation between B. burgdorferi FliR and homologs from other bacteria

  • Investigation of spirochete-specific adaptations in the flagellar system

  • Cross-complementation experiments

• Integration with whole-cell studies:

  • Correlation of in vitro findings with flagellar structure observed by cryo-electron tomography

  • Assessment of how mutations affect flagellar number, length, and function

  • Investigation of the temporal sequence of flagellar assembly in live cells

By combining these approaches, researchers can develop comprehensive models of flagellar assembly in spirochetes, which differ significantly from model organisms like E. coli or Salmonella due to their periplasmic flagella and unique cellular architecture.

Citations Mutations in the Borrelia burgdorferi Flagellar Type III Secretion System Components FliH and FliI Reveal Their Roles in Flagellar Formation, PMC4436065, 2015. Combined Effects of Blood and Temperature Shift on Borrelia burgdorferi Gene Expression as Determined by Whole Genome DNA Array Analysis, PMC517457, 2004. Deciphering the Ligand-binding Sites in the Borrelia burgdorferi Complement Regulator-acquiring Surface Protein 2 Required for Interactions with the Human Immune Regulators Factor H and Factor H-like Protein 1, PMC2596382, 2008. People-finding with a Lepton | Teledyne FLIR, flir.com, thermal camera technology guide. Borrelia burgdorferi σ54 is required for mammalian infection and vector transmission, PNAS, 2005.